Method of coating copper plated strands with zinc

ABSTRACT

A METHOD AND APPARATUS FOR CONTINUOUSLY APPLYING A MOLTEN COATING METAL TO A STRAND-LIKE ARTICLE. THE METHOD INCLUDES THE STEPS OF CLEANING THE SURFACE OF THE STRAND AND PREPARING IT TO RECEIVE THE MOLTEN METAL, AND THEN SUPPLYING AN EXCESS OF MOLTEN METAL TO THE MOVING STRAND. THE APPARATUS HAS A COATING CHAMBER THROUGH WHICH THE STRAND PASSES IN A HORIZONTAL DIRECTION. THE CHAMBER HAS SUBSTANTIALLY UN-RESTRICTED ENTRY AND EXIT OPENINGS, AND INCLUDES A RESERVIOR AND PASSAGE MEANS FOR SUPPLYING AN EXCESS OF THE COATING METAL TO THE MOVING STRAND.

METHOD OF COATING COPPER PLATED STRANDS WITH ZINC Filed Dec. 12, 1968 V 46 m A INVENTORS KENNETH 0. COBURN a ATTORNEYS 3,597,261 METHOD OF COATING COPPER PLATED STRANDS WITH ZINC Kenneth G. Coburn, Middletown, Ohio, Ralph E. Evans,

Denver, Colo., and Marvin B. Pierson, Middletown, Ohio, assignors to Arrnco Steel Corporation, Middletown, Ohio Continuation-impart of application Ser. No. 616,680,

Feb. 16, 1967. This application Dec. 12, 1968,

Ser. No. 783,266

Int. Cl. Bc 11/06; C23c N02 US. Cl. 117-71 3 Claims ABSTRACT OF THE DISCLOSURE A method and apparatus for continuously applying a molten coating metal to a strand-like article. The method includes the steps of cleaning the surface of the strand and preparing it to receive the molten metal, and then supplying an excess of molten metal to the moving strand. The apparatus has a coating chamber through which the strand passes in a horizontal direction. The chamber has substantially tin-restricted entry and exit openings, and includes a reservoir and passage means for supplying an excess of the coating metal to the moving strand.

CROSS REFERENCE TO RELATED APPLICATIONS This application is a continuation-in-part of application Ser. No. 616,680 filed Feb. 16, 1967, and now abandoned in the names of Coburn, Evans, and Pierson and entitled Method and Apparatus for Coating Metallic Strands.

BACKGROUND OF THE INVENTION It will be understood that as used in this specification, the word strand is used to include materials that have a generally circular cross section, such as wire, tubing, and the like.

This invention has great and particular utility in the coating of Bundy type tubing. As is Well known in the art, Bundy tubing is a double wall copper brazed tubing, utilized in handling liquids under pressure where absolutely no leakage can be tolerated.

Commercially available Bundy tubing is formed by rolling a copper plated low carbon steel strip into a double walled tube. By the application of heat, the Walls are brazed together, with the copper acting as the brazing material. The copper plating on the starting material is extremely thin, and provides no significant corrosion resistance.

It has long been realized that for various applications such as automobile brake lines, a more corrosion resistant tubing would be highly desirable.

On the one hand, the prior art has attempted to achieve this corrosion resistance by cladding the tubing with various metals such as a lead-tin alloy. This product does not have desirable corrosion resistance, in chloride atmospheres, for example.

On the other hand, the galvanizing or coating of a ferrous strand with molten metal is a highly developed art, and prior efforts to coat Bundy tubing according to conventional practices have proved unsuccessful. It is believed that this lack of success is due primarily to two causes.

In the first place, this invention contemplates a coating method and apparatus wherein the strand is moving in a horizontal path of travel. It will be understood by the skilled worker in the art that the rigid character of tubing will not permit the use of the more conventional vertical 3,597,261 Patented Aug. 3, 1971 coating equipment. Present methods for the horizontal coating of strand-like articles require the use of a restricted die through which the strand passes into the bath of the molten coating metal. As will be well understood in the art, these hot dip metallic methods require the stock to reach the melting temperature of the coating metal being used, which in the case of zinc is on the order of 800 F. At these temperatures the copper plating on the Bundy tubing becomes prone to galling, and tends to build up and accumulate at this restricted die opening. Actual experiments have disclosed that in a very short period of time (less than one minute), the accumulation of copper becomes so great that the strand being coated is pinched off.

In the second place, it has been found that the copper plating on the steel tubing tends to dissolve in the molten coating metal. This alteration in the composition increases the melting point of the coating metal and seriously affects the appearance of the finished product. In addition, the accumulation of copper in the molten zinc bath greatly increases the rate of deterioration of the ferrous portions of the coating equipment.

It is therefore a primary object of this invention to provide a coating method which does not utilize a restricted orifice in the high temperature region of the equipment.

A second very important object of this invention is to provide a coating method wherein the immersion time of the material to be coated in the coating bath is extremely short.

It will be recognized by the skilled worker in the art that this last mentioned general objective will be of great utility in many other metallic coating applications and as indicated above, the frequent reference in this specification to the coating of Bundy tubing is not to be construed as a limitation of the invention.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of the process of this invention;

FIG. 2 is a schematic view of the coating compartment of this invention;

FIG. 3 is a cross sectional view of the preferred apparatus for the practice of this invention; and

FIG. 4 is a cross sectional view taken along the line 44 of FIG. 3.

SUMMARY OF THE INVENTION Briefly considered, this invention contemplates a hot dip method of coating a ferrous strand with any of the conventional coating metals. The method includes the first step of preparing the surface of the strand to receive the molten coating metal. The strand is then passed into a coating compartment in which a protective atmosphere is maintained and which is free of a restrictive entry opening, and an excess of molten coating metal is supplied to the moving strand. As the strand emerges from the coating compartment, the excess coating metal is removed, and the metallic coating is solidified.

DECRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1, the strand to be coated is indicated at 10. As indicated above, the first step of the invention involves the preparation of the strand to receive the molten coating metal. As is well known in the art, this involves a thorough cleaning of the surface of the strand. In the embodiment shown, this cleaning is accomplished by passing the strand through a suitable alkaline cleaning compartment 12. In the compartment 12, a suitable alkaline solution can be sprayed onto the moving strand.

Upon leaving the alkaline compartment 12, the strand is preferably rinsed by the spray 14, and passed into an acid pickle compartment 16. The various parameters of the temperature and acid bath composition are Well known in the art, and do not, per se, form a part of the instant invention. As the strand leaves the acid pickle compartment 16-, it is advantageous to remove the pickle residue. This may be accomplished by a hot water rinse as indicated at 18.

It will be understood that the above described chemical cleaning section is simply exemplary of various well known expedients. For example, the chemical cleaning section may be replaced by a lead oxidizing bath, or by simply heating the strand in an oxidizing atmosphere. Similarly, the rinse 18 may under certain conditions be replaced by a mechanical scrubbing operation if desired.

The cleaning operation is completed by heating the strand in a protective atmosphere. In the embodiment shown, the strand 10 passes over a suitable guide sheave 20, and into the tube 22 which is mounted within the furnace 24. A suitable gas, such as dissociated ammonia, will be introduced into the tube 22 from an inlet not shown in the drawings, so as to provide a reducing atmosphere. The temperature of the furnace 24 will of course vary, depending upon the coating metal used. In an exemplary operation utilizing a zinc coating, the tube may be annealed at 1200 F. or higher, and then cooled to 800- 900 F. prior to coating.

Without re-exposing the strand 10 to atmosphere, it will passfrom the tube 22 into the coating compartment indicated in FIG. 1 at 26, wherein an excess of molten coating metal is supplied to the moving strand. As briefly indicated earlier, in this specification, it should be noted that the strand 10 passes into the coating compartment 26 without going through a restrictive entry die or opening of any sort after the strand reaches coating temperature.

It will be seen that the strand 10 passes through the exit opening of the coating compartment with substantial clearance. However, it is contemplated that the excess coating metal supplied to the strand will be sufhcient to seal the exit opening of the coating compartment.

As the strand emerges from the coating compartment 26, the excess coating metal is removed from the strand by conventional means, such as the jet finishing nozzle indicated generally at 28.

As is well known in the art, it is desirable to solidify the coating and cool the strand as soon as possible after emerging from the coating compartment. To this end, a suitable quenching compartment as indicated at 36 will be provided. Water is preferred and may contain additions such as chromic acid to provide protection against white rusting in the case of zinc.

Turning now to FIG. 2, one embodiment of the coating compartment indicated generally at 26 will be described in more detail. It will be observed in this figure that the strand being coated is again indicated at 19. FIG. 2 also shows a portion of the tube 22 in which the strand 10 is heated in a reducing atmosphere.

The coating compartment includes a housing portion indicated at 32, a supply reservoir 34, an overflow and melt reservoir 36, and heating means 44 to keep the metal molten. A molten metal pump 38 maintains a supply of coating metal to reservoir 34 at a rate equal to the rate of discharge through the spout 41.

It will be seen that one wall of the housing portion 32 is secured as by welding or the like to one end of the tube 22. The upper end of the housing portion is sealingly secured to the bottom surface of the supply reservoir 34, while the lower, and open end of the housing 32 is immersed in the molten metal contained in the overflow reservoir 36. It will also be observed that there is no restricted entry die of any sort between the tube 22 and the housing 32 of the coating compartment.

The supply reservoir .34 will be filled with molten coating metal to approximately the level indicated by the line 40. The pump discharge should be located below the surface of the melt to minimize turbulence and concomitant oxidation of the melt. This molten coating metal will flow by gravity through the downwardly extending spout 41 onto the passing strand 10. Spout 41 should preferably be directed at the exit orilice 42. Movement of the strand combined with the location of molten metal application results in a very short immersion time or length. This is a very important facet of this invention since this is the key to minimizing copper solution in the coating metal. When coating /8 inch tubing for example, only about 1 inch of the tubing is in contact with the cascading coating metal.

The size of the spout 41 and the quantity of molten metal contained in the reservoir 34 must be such that an excess of coating metal is poured onto the moving strand. A conventional pump and valving means can also be utilized to control metal flow. As indicated at the outset of this specification, this invention is applicable to the coating of strand-like articles which are substantially circular in cross section. It will be understood that the term excess as used in this application simply means that a greater quantity of coating metal is poured onto the strand than is carried out of the coating compartment by the strand. The supplying of an excess of molten coating metal to the strand is necessary in order that the molten metal can cover the underside of the moving strand, and seal the exit orifice 42. This excess may be controlled by needle valve 43 or a similar device, so that the excess is maintained without unnecessary washing of copper from the strand or unnecessary circulation of the coating metal which will produce serious oxidation losses.

It will be recalled that one object of this invention was to limit the change of composition of the coating metal. That is, for example, in the coating of copper plated tubing, the molten zinc will dissolve a portion of the copper plating, this dissolved copper being carried with the cascading zinc into the melt reservoir 36. In the embodiment of the invention utilized in the coating of such copper plated tubing, it will be necessary to add fresh zinc to the melt reservoir 36, in an amount substantially equal to the zinc removed by the strand being coated. It has been found that the pour on technique described above results in an extremely short immersion time of the moving strand in the molten metal, and hence the amount of the dissolved copper carried into the melt reservoir 36 is very small.

The addition of the requisite amount of fresh coating metal balanced against the rate of solution of copper will stabilize the amount of copper in the zinc coating bath at less than 1%. The equilibrium percentage of copper in the zinc bath will increase with longer immersion and will decrease with increasing coating weights.

It will be apparent that the molten metal pump 38 is utilized to convey the molten material from the reservoir 36 into the supply reservoir 34.

As explained above, an excess of molten coating metal is poured onto the moving strand in order to insure coverage of the entire surface of the strand. As the strand emerges from the coating compartment, it is necessary to remove this excess metal. In the embodiment shown, the removal of excess coating metal is accomplished by the jet finishing nozzle 28, which serves to direct a fluid stream against the coated strand. Various fiuids may be used, such as steam, air or reducing gases.

As is well known in this art, it is necessary to control the atmosphere in the tube 22, as well as in the coating compartment by being secured to the tube 22. The exit orifice 42 of the housing 32 will preferably be sealed by the excess of coating metal poured onto the strand.

As soon as possible after emerging from the jet finishing nozzle 28, the coating must be solidified to prevent sagging of the molten coating and to minimize alloying of the coating with the base metal. This can be accomplished in any conventional manner, as by the quenching compartment 30 briefly noted above.

FIGS. 3 and 4 illustrate another embodiment of apparatus for practicing this invention. In point of fact, due to its great simplicity, this embodiment is to be preferred in many applications.

The tube 22 mounted within the furnace 24 is substantially similar to the earlier embodiment, except that in this case it is provided with the zinc return passage 22a shortly before its outer-most end. Fixedly secured to the end as by welding or the like is the coating applicator 46 which will be formed of any suitable material such as type 316 Stainless Steel.

It will be seen in FIG. 3 that the generally cylindrical coating applicator 46 is provided with an axial passage including the exit hole 48, the entry hole 50, and the bell 52. The bell 52 is simply designed to accommodate threading of the materials to be coated through the applicator.

By comparison, the relative diameters of the entry and exit holes 50 and 48 respectively are important. As explained throughout this application, it is imperative that the tubing to be coated pass into the coating compartment without a restricted entry opening. Thus, the entry hole 50 must have substantial clearance about the strand being coated. In an exemplary embodiment, in which /8 inch Bundy tubing is being coated, the entry hole will have a diameter on the order of .430 inch.

It will be seen from the drawing that the exit hole 48 is slightly smaller in diameter. In the same embodiment of the invention, an exit hole diameter on the order of .400 inch has proved satisfactory. While an opening of .400 inch about a strand inch in diameter provides substantial clearance, it is again contemplated that the excess of molten coating metal applied to the strand will seal the exit opening.

It will be seen in FIG. 4 that the coating applicator 46 is provided with a horizontal blind bore 54 which intersects the axial passage through the applicator. This passage in effect forms the coating compartment within the center of the applicator 46.

It will of course be understood that the passage 54 will be connected to a suitable supply of molten zinc. Since this connection is entirely conventional, it is not shown in the drawings.

With the arrangement just described, it has been discovered that the strand will be wet immediately by the molten zinc in the passage 54, so that its surface is com pletely and thoroughly covered. By virtue of the smaller clearance between the strand being coated and the exit hole 48, the exit hole actually serves to center the strand in the entry hole, and positively insures that no contact can take place between the strand and the entry hole. Thus, the foregoing construction absolutely eliminates the problem of galling caused by copper accumulation at the entrance opening to the coating compartment.

It will be understood that various modifications may be made in this invention without departing from its scope or spirit. Accordingly, no limitations on this invention are intended except as specifically set forth in the claims which follow.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. The method of coating a copper plated strand with molten zinc comprising the steps of:

(a) moving said strand in a horizontal path of travel;

(b) thoroughly cleaning the surface of said strand and heating said strand in a protective atmosphere;

(c) exiting said strand from said protective atmosphere through an orifice with substantial clearance about said strand;

(d) sealing said clearance about said strand with a flow of an excess of molten coating metal;

(e) thereafter removing the excess coating metal; and

(f) solidifying said metallic coating.

2. The method claimed in claim 1 wherein said removal of excess coating metal is accomplished by directing a fluid stream against said coated strand.

3. The method claimed in claim 2 including the steps of returning said removed excess coating metal to said supply and adding fresh coating metal in such proportion that the amount of dissolved copper in said supply is maintained at less than 1%.

References Cited UNITED STATES PATENTS 287,076 10/1883 Young 117-102(M) 370,710 9/1887 Matteson 117 Air Blast 1,605,326 11/1926 Bundy 118-325X 1,981,130 11/1934 Underwood 117-102(M) 2,197,622 4/ 1940 Sendzimir 11751 2,374,926 5/1945 Fink 1171 l4 BuX 2,683,099 7/1954 Hahn 117102(M)X 2,876,132 3/1959 Worden et a1 1l794X 2,914,423 11/1959 Knapp 1l7102(M) ALFRED L. LEAVITT, Primary Examiner J. R. BATTEN, 1a., Assistant Examiner US. Cl. X.R. 

